Motion pictures, video and digital photography use compositing to alter image quality or create visual effects by combining images from different sources. Images and colors may be altered by manipulating pixels through color conversion, keying, scaling and blending operations. Compositing is used extensively in modern film, television and photography to achieve effects that otherwise would be difficult or not cost-efficient. One common use for compositing is to shoot on a relatively small set and create the impression of a significantly different location by adding additional surrounding and foreground imagery. A common tool to help facilitate composites is using a blue or green backdrop of a uniformly solid color that is placed behind an actor or object. During compositing, all areas of a frame with that color are removed and replaced, allowing the compositor to place the isolated image of the actor or object in front of a separately shot or synthetic background.
Compositing technique is commonly done using digital techniques. One of the known digital techniques is a raster operation (ROP) where changes of image graphics may be transitioned smoothly by compositing the source image data to the destination image data through programming a ROP table. A digital image may be reproduced by clustering pixels or dots of varying intensity and color, forming a raster graphics image. A colored raster graphic image will usually have pixels with between one and eight bits for each of the basic colors red, green, and blue components (R, G, B). By using red, green and blue intensities, many color combinations can be represented. Typical display adapters may use up to 24 bits of information for each pixel. This is usually apportioned with 8 bits each for red, green and blue, giving a range of 256 possible values, or intensities, for each hue. With this system, (2563 or 224) discrete combinations of hue and intensity may be specified.
With the need for compositing images comes a variant of RGB which includes an extra 8 bit channel for transparency, thus resulting in a 32 bpp format. The transparency channel is commonly known as the alpha channel, so the format is named RGBA. Since the RGBA does not change anything in the RGB model, it is not a distinct color model. The RGBA is a file format which integrates transparency information along with the color information in the same file. This allows for alpha blending of the image over another, and is a feature of the PNG format.
In graphics and visual effects, keying is an informal term for compositing two full frame images together, by discriminating the visual information into values of color and light. Some of the common key types: chroma key, luma key, difference key and matte key. A chroma key is the removal of a color from one image to reveal another “behind” it. A luma key similarly replaces color from an image which falls into a particular range of brightness. This technique is less controllable, but may be used on graphic elements. It is particularly useful for realistic fire keying. A difference key uses a background plate of the scene that the foreground object is being keyed out of and the software then assesses the source video and any pixels that don't match the grid are obviously meant to be keyed out. For example, if your subject is standing in front of a wall, a photo taken from the camera of the same wall is used. This may be taken from the same camera angle, focus & distance. The software may then compare the video to be keyed with the original photo and generates a mask based upon that difference.
The quality of a raster graphic image is determined by the total number of pixels (resolution), and the amount of information in each pixel (often called color depth). For example, an image that stores 24 bits of color information per pixel can represent smoother degrees of shading than one that only stores 16 bits per pixel. Likewise, an image sampled at 640×480 pixels (therefore containing 307,200 pixels) will look rough and blocky compared to one sampled at 1280×1024 (1,310,720 pixels).
Because it takes a large amount of data to store a high-quality image, data compression is used to reduce the memory size and may result in some loss of resolution in raster graphics image. The amount of memory space used by an uncompressed image is specified by the number of pixels in the image and the color depth to which each pixel may be specified. In a 24-bit image, each pixel may be specified by a 24-bit allocation of memory, so the amount of space required in bits is 24× the number of pixels. For example, a 24-bit image 640×480 pixels in size requires (24×640×480)/8=921,600 bytes of memory.
Compositing two images of different sizes require scaling of one image to the other. Scaling is a non-trivial process that involves a trade-off between speed, smoothness and/or sharpness. Image scaling may involve either scale down by sub-sampling or zooming out (reducing or shrinking an image) or scale up by zooming in (enlarging an image). When increasing or reducing the size of images, the problem of processing efficiency becomes particularly evident because of the fact that digital images are composed of large number of pixels.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.